1. Field of the Invention The present invention relates to a coding apparatus.
2. Related Background Art
Generally, in the case of magnetically recording a signal of a high bit rate such as a digital video signal, highly efficient coding to compress a data transmission band is executed, and to further reduce a code error ratio, a DC suppressing process to eliminate components of a very low frequency from a recording signal is executed.
As an example of a highly efficient coding, a predictive difference coding (hereinafter, referred to as a DPCM) for coding a difference value between adjacent sample values has been known. Further, as a DC suppressing process, a mapping coding for suppressing the DC component by converting a code of n bits into another code of m bits has been known.
As a method of efficiently executing such highly efficient coding and DC suppression as mentioned above, the assignee of the present invention has proposed a coding method wherein when a difference value is coded, a code of a small CDS (Codeword Digital Sum) value is assigned to any difference value of a high occurrence frequency (Japanese Laid-Open Patent Application No. 61-99419). On the other hand, as a method of further efficiently executing the DC suppression, the assignee of the present invention has proposed a coding method wherein a plurality of difference values are converted into codes of a suppressed DC component (Japanese Laid-Open Patent Application No. 63-33086).
By using those methods, the DC component can be suppressed without increasing redundancy of the code series.
Now, considering the case where the redundancy is not increased by the above method, the code after completion of the code conversion includes a certain DC component, in dependence on the pattern of the difference codes. Therefore, particularly, in the case where the DC component must be suppressed to an extremely small level, for instance, it is difficult to apply such a method to a coding apparatus for DPCM coding a high definition television signal (hereinafter, referred to as an HD signal).
On the other hand, when a television signal is DPCM coded and magnetically recorded, actually, codes (PCM codes) in which the sample values have been directly quantized to prevent the error propagation of DPCM, check bits of an error correction code, and other codes such as sub-codes, exist, and therefore the DC component of such codes must be also suppressed. However, according to the foregoing method, it is difficult to suppress the DC component of such other codes.